Over the years, Nd—Fe—B sintered magnets find an ever increasing range of application including hard disk drives, air conditioners, industrial motors, power generators and drive motors in hybrid cars and electric vehicles. When used in air conditioner compressor motors, vehicle-related components and other applications which are expected of future development, the magnets are exposed to elevated temperatures. Thus the magnets must have stable properties at elevated temperatures, that is, heat resistance. The addition of Dy and Tb is essential to this end whereas a saving of Dy and Tb is an important task when the tight resource problem is considered. For those magnets of the relevant composition which are expected to find ever increasing applications, it is desired to reduce the amount of Dy or Tb to a minimal level or even to zero.
For the relevant magnet based on the magnetism-governing major phase of Nd2Fe14B crystal grains, small domains which are reversely magnetized, known as reverse magnetic domains, are created at interfaces of Nd2Fe14B crystal grains. As these domains grow, magnetization is reversed. In theory, the maximum coercivity is equal to the anisotropic magnetic field (6.4 MA/m) of Nd2Fe14B compound. However, because of a reduction of the anisotropic magnetic field caused by disorder of the crystal structure near grain boundaries and the influence of leakage magnetic field caused by morphology or the like, the coercivity actually available is only about 15% (1 MA/m) of the anisotropic magnetic field. Although this coercivity is of low value, the presence of a Nd-rich phase surrounding crystal grains is essential to develop such a value of coercivity. Therefore, in preparing sintered magnets, an alloy composition containing rare earth element in excess of the stoichiometric Nd content (11.76 at %) of Nd2Fe14B compound is used. Although part of excessive rare earth element acts as a getter for oxygen and other impurity elements which are incidentally introduced during the preparation process, the majority surrounds major phase crystal grains as a Nd-rich phase and contributes to development of coercivity. Further, since the Nd-rich phase is liquid at the sintering temperature, the relevant composition magnets undergo further consolidation via liquid phase sintering. This indicates sinterability at a relatively low temperature, and the presence of a hetero-phase at grain boundaries is effective for suppressing major phase crystal grains from growing.
It is empirically known that a magnet of the above composition is increased in coercivity by reducing the size of Nd2Fe14B particles as the major phase while maintaining the crystal morphology of the composition. The method of preparing a sintered magnet includes a finely pulverizing step, through which a magnet material is typically pulverized into a powder with an average particle size of about 3 to 5 μm. If the particle size is reduced to 1 to 2 μm, then the crystal grains in the sintered body are also reduced in size. As a result, the coercivity is increased to about 1.6 MA/m. See Non-Patent Document 1.
In fact, apart from the sintered magnets, Nd—Fe—B magnet powders, which are prepared by the melt quenching process or HDDR (hydrogenation-disproportionation-desorption-recombination) process, are composed of submicron crystal grains with a grain size of up to 1 μm. Some of them exhibit a higher coercivity than the sintered magnets when compared for the Dy or Tb-free composition. This fact suggests that size reduction of crystal grains leads to an increase of coercivity.
The only one means for obtaining such submicron crystal grains in the sintered magnet which has been discovered thus far is to reduce the powder particle size during the finely pulverizing step as reported in Non-Patent Document 1. If Nd—Fe—B alloy is pulverized into a fine powder, the powder is liable to oxidation because of highly active Nd, even with the danger of ignition. When magnet manufacture is carried out under such conditions as to have an average particle size of 3 to 5 μm, a suitable measure is taken for the duration from the fine pulverizing step to the sintering step. For example, the atmosphere is filled with an inert gas to avoid contact with oxygen, or the fine powder is mixed with oil to avoid contact of the powder with the ambient air. However, the particle size that can be reached by fine pulverization is limited to the order of 1 μm, and no guideline for obtaining crystal particles finer than this limit is available in the art.
On the other hand, the above-mentioned HDDR process is intended to gain a coercivity by heating a cast Nd—Fe—B alloy in hydrogen atmosphere at 700 to 800° C., and subsequently heat treating in vacuum, thereby changing the alloy structure from the crystal grains in the cast alloy having a size of several hundreds of microns (μm) to a collection of submicron crystal grains having a size of 0.2 to 1 μm. In the HDDR process, the Nd2Fe14B compound as major phase undergoes disproportionation reaction with hydrogen in the hydrogen atmosphere, whereby it disproportionates into three phases, NdH2, Fe, and Fe2B. Via the subsequent vacuum heat treatment for hydrogen desorption, the three phases are recombined into the original Nd2Fe14B compound. During the process, submicron crystal grains having a size of up to 1 μm are obtainable. Also, the HDDR process enables size reduction, depending on a particular composition or processing conditions, while the crystallographic orientation of submicron crystal grains is kept substantially the same as the crystallographic orientation of initial coarse crystal grains. Thus an anisotropic powder with a high magnetic force is obtainable. However, generally a hetero-phase (compound phase of heterogeneous composition) which is wider than a certain value (e.g., a width of at least 2 nm) does not exist between submicron crystal grains. This allows for grain growth to readily take place if the heat treatment temperature for recombination is high only slightly. Then high coercivity is not available. Although the HDDR powder is typically mixed with resins to form bonded magnets, an attempt to form a full-dense magnet has been made to produce a high magnetic force equivalent to sintered magnets. Most research works utilize the hot pressing step of compressing the powder while applying heat at substantially the same temperature as the HDDR process temperature, as described in Patent Document 1. However, this process has not been implemented in the industry because of extremely low productivity.
Other attempts are known from Non-Patent Document 2, for example, brief sintering by electric conduction sintering and sintering of a dense mass which is obtained by consolidating the HDDR powder in a rotary forging machine. Allegedly, the electric conduction sintering results in a variation in density of a sintered body, and the forging/sintering process allows for significant grain growth. It is thus believed difficult to form a full-dense magnet by sintering the HDDR powder.